In general it is known to prepare polymers in a liquid solution. However, such methods require in many cases the use of expensive, corrosive or toxic solvents. Further, often undesired or uncontrollable reactions take place in such a solution. Finally, after polymer formation in the liquid solution, a film or coating has to still be formed which normally requires multiple further time-consuming cleaning and deposition steps.
Another known method allowing a direct deposition of polymer films is the method of chemical vapor deposition (CVD).
One type of CVD is the method of vapor deposition polymerization (VDP). According to VDP a condensation reaction is initiated between two monomers at a surface of a substrate without using any initiator or oxidizing agent. In particular, monomers are volatilized at the surface of a temperature controlled substrate. Such a method allows polymerization of monomers which are not soluble in solvents and offers to some degree a control of generated chain lengths. However, according to the VDP it is difficult to control the monomers' concentration and their flux to the surface of the sample to be coated. In particular, there is a competition between an adsorption on the sample's surface and the formation of long polymer chains, wherein adsorption is preferred at low temperatures and the formation of long polymer chains is preferred at rather high temperatures. This is only one of the reasons for the difficulty in controlling the composition of copolymers according to this method.
Another known polymerization method is the plasma enhanced chemical vapor deposition (PECVD). This technique has mainly been used for the deposition of inorganic films at relatively low temperatures and with a high deposition rate. Unfortunately, monomers injected into the plasma chamber are ionized or fragmentized by the plasma due to electrons, ions or radicals in the plasma. Further, such plasma polymers are often irregular and have rather short chain lengths. Moreover, they are frequently cross-linked and randomly terminated. Plasma polymerization leads to a random poly-recombination of radicals and fragments of monomers. A known innovation in the field of plasma polymerization is the use of the pulsed-plasma mode. In that case, short plasma pulses (of a few microseconds) can activate molecules, produce radicals and initiate the polymerization reaction. After the pulse, the residual radicals initiate a purely chemical radical chain reaction during the plasma-off period (typically tens of microseconds). In that case, a more chemically-regular product is expected but the chain propagation during chemical polymerization is restricted by the low probability of attaching a new monomer to the radical at the growing chain-end. Moreover, the deactivation of chain propagation is expected under the usual vacuum conditions (e.g. about 10 Pa) due to the recombination of neighboring radicals. This leads to a significant loss of active radical sites during the plasma-off period. Plasma is thus re-ignited to reproduce fresh initiating radicals. The resulting polymer films have a structure and composition closer to their counterparts produced by radical polymerization in liquid phase but show repeatable irregularities induced by the plasma-on period. Such issues should be circumvented.
Another known method for depositing polymer coatings is the initiator chemical vapor deposition (iCVD). This method is based on a thermal decomposition of an initiator into free radicals assisting in initiating the polymerization of co-injected monomers. Typically the iCVD uses a network of heated filaments (e.g. between 200° C. and 550° C.) such that the initiator molecule is pyrolyzed into radicals capable of initiating a polymerization reaction. Then the radicals diffuse together with the monomers to a carrier substrate. Unfortunately, pyrolysis may also result in the degradation of monomers and is difficult to control. Moreover, pyrolysis of initiator material may result in further undesired ions or agents which may deteriorate the quality of the produced polymer and impede the control of polymer formation. Further, the amount of radicals and monomers reaching a substrate's surface is difficult to control. In other iCVD methods it is possible that initiators are activated by UV radiation (photo initiator CVD). However, all known iCVD methods do not allow for sufficient control of the polymerization reaction. In particular, free radicals recombine uncontrollably before deposition.
Finally, the method of oxydant chemical vapor deposition (oCVD) is known in the state of the art. In particular, a monomer reacts with an oxidant species. The oCVD uses a spontaneous reaction between the oxidant and the monomer with any energetical initiation (e.g. thermal or optical). However, the oxidants have a very low volatility and their injection into the vapor phase is a difficult problem requiring very specific reactor types.
As indicated above, known methods involve several disadvantages. In summary, they often do not allow for a satisfying control of chain lengths of formed polymers. Another issue is the degradation of polymers and/or polymer forming material which can result in the loss of the desired functional properties of the produced polymer coating. In particular, some of the above methods utilize a plasma within the reaction chamber which results in the degradation of sensitive polymers or precursors, i.e. in particular of organic material.
Moreover, it is not always possible to sufficiently control the concentration of polymer forming material in relation to initiators, thus rendering the control of layer thicknesses and chain lengths difficult.
Other methods involve multiple serial coating steps and are thus time consuming, rendering these methods rather uninteresting for mass production purposes.
The technical problem is to provide an advanced method of forming polymer coatings on substrates.
In particular, the method should allow for an improved control of polymer chain lengths, cross linking and or coating thickness.
Further, the method should be fast, easy to implement or cost-effective.
Another object of the invention is to overcome at least one of the above mentioned disadvantages.